1. Field of the Invention
Exemplary aspects of the present invention relate to an image forming apparatus, and more particularly to an image forming apparatus and the changing of the heating width of its fixing device.
2. Description of the Related Art
In general, image forming apparatus of recent years such as copiers, printers, facsimile machines, and multifunctional peripherals employ an electro-photographic method to form a toner image on an image carrier by a series of image forming processes such as charging, writing, developing, transferring, cleaning, and discharging. The toner image on the image carrier is transferred on a recording medium such as transfer paper and overhead projector films. The image forming apparatus includes a fixing device in which heat and pressure are applied by a fixing member and a pressure member to fix the toner image on the transfer medium.
Such image forming apparatuses are demanded to shorten a warm-up time when power is initially turned on. The fixing member of the fixing device increases the temperature thereof to a suitable level during warm-up so that the image forming apparatus can become operatable.
In attempting to shorten the warm-up time, fixing devices having a fixing member with a relatively low heat capacity have been developed and practically used. Specific examples thereof include a fixing device in which a nip is formed between a rotation member and a fixing film that has a relatively low heat capacity and faces the rotation member. Another example is a fixing device that employs a fixing belt as the fixing member, wherein and the fixing belt is tightly stretched by a plurality of rotation members. One of the plurality of rotation members has a relatively low heat conduction and forms a fixing nip with the a rotation member while another of the plurality of rotation members includes a heat source such as heaters. Recently, induction heating systems in which a fixing member having a relatively low heat capacity is directly heated have been used as the heat source as well as heaters. The fixing devices with the induction heating system have an advantage such that the fixing member having a relatively low heat capacity can increase the temperature thereof in a relatively short time period, resulting in shortening of the warm-up time of the fixing device. One example attempts to shorten the warm-up time by applying a relatively large amount of heat to the fixing belt in a short time period. The fixing device includes a contact member which presses the fixing belt so that the fixing belt and a heating roller having a heat source therein have a large contact length. Thereby, a relatively large amount of heat is applied to the fixing belt in a short time period.
According to another example of an attempt to reduce fixing errors when a variety of recording media having different widths are used, an increase in temperature at both ends of the fixing member in a width direction is controlled even when a plurality of recording media having small widths are successively fixed. In addition, the fixing device includes an induction heating unit including a shielding member for shielding and a core. The shielding member varies the heating area in a width direction of the fixing member by shielding a part in a width direction of the core when an electric current is applied to a coil of the induction heating unit. The core includes a protrusion member at the end in the width direction thereof. The protrusion member protrudes towards the coil of the induction heating.
Therefore, the related art fixing device attempts to reduce the increase in temperature of the fixing member at a non-sheet feeding area, by controlling the width of the heated portion of the fixing member when a recoding medium having a relatively small width is fed.
However, when the width of the recording medium is frequently switched for one printing operation, the control of the heating width is late due to the response speed of the mechanical operation or the response speed by the heat capacity of the fixing member. Thereby, the heating width is not properly controlled. When a related art heat fixing device with a low-speed and low-heat amount and a high-speed and high-heat amount is used, the temperature of a non-sheet feeding area cannot reach a risk range immediately because the control of the heating width is late.